1. Field of the Invention
The present invention relates to an internal combustion engine start controller, and more particularly relates to an internal combustion engine start controller in a vehicle provided with an internal combustion engine that is started by using the electric power supplied from a power source.
2. Description of the Related Art
An internal combustion engine mounted on a vehicle is generally started by a motor (starter motor) that is driven by the electric power supplied from a power source. In particular, a configuration in which the engine is started by an AC motor that is driven and controlled by the AC voltage supplied from an inverter that is obtained through switching control of power semiconductor elements.
For example, Japanese Patent Application Publication No. 2004-183560 discloses a vehicular engine start controller in which a single AC motor starts the engine and generates electricity after the engine has been started. With this vehicular engine start controller, it is possible to reduce the number of motors as compared to a configuration including both a DC motor for starting the engine and an AC motor used only for generating electricity. In addition, because there is no fear that the commutator wears, it is possible to improve the reliability of idle-stop vehicles that involve frequent engine start-ups.
With regard to the control of the inverter for driving a motor, available are a controller that changes the carrier frequency used to control the inverter according to the temperature of power semiconductor elements (power elements) constituting the inverter (see Japanese Patent Application Publication No. 5-115106), and a voltage converter for driving and controlling an inverter by increasing the carrier frequency so as to prevent the impairment of the operational characteristics of the inverter when the atmospheric temperature drops and the counter electromotive force of the motor exceeds the withstand voltage of the inverter (see Japanese Patent Application Publication No. 2004-166341, for example).
However, because the electric power conversion performed by an inverter causes the power loss (switching loss) at the time of switching operation, there is a possibility that, during engine startup that requires a relatively large current, the required large current together with the power loss in the inverter causes power consumption to increase, and therefore causes the power source to be overloaded.
In particular, if a secondary battery is used as the power source, it becomes difficult to supply high electric power because the power density of the battery decreases under a low temperature condition. In addition, under a low temperature condition, the frictional torque in an internal combustion engine increases, and therefore, the electric power required to start the engine further increases. Consequently, there is a possibility that engine startability is impaired because the load on the power source becomes excessive and the output of the power source thus decreases. If the design margin of the power source capacity is increased in order to ensure engine startability, the size and cost of the power source can be increased.
With this regard, although Japanese Patent Application Publication No. 2004-183560 discloses a configuration in which the engine is started by the AC motor driven and controlled by an inverter, it makes no mention of how to address the problem that the load on the power source applied during engine startup increases.
Similarly, although Japanese Patent Application Publication No. 5-115106 and Japanese Patent Application Publication No. 2004-166341 disclose a control method in which the carrier frequency of an inverter is changed according to the temperature of power semiconductor elements or the atmospheric temperature, these documents make no mention of how to address the problem that the load on the power source applied during engine startup increases.